Neutron Activation Analysis

Neutron Activation Analysis (NAA) is one of the most sensitive methods used to measure the concentration of trace amounts of many elements in a variety of sample types. In NAA, a sample is bombarded with neutrons, resulting in the production of a radioactive isotope of the element of interest. Gamma rays emitted by the radioactive isotope are then analyzed. The energy associated with the radiation is characteristic of the radioactive isotope, and hence it is used for element identification i.e., qualitative analysis. The number of gamma rays emitted is correlated to the number of atoms present in the sample, i.e., quantitative analysis.

NAA is a microanalytical method; Generally, only a few milligrams of sample are required. This is of value in examination of lunar samples and artifacts, and in criminal investigations. NAA is a nondestructive technique, where analysis can be performed without destroying the sample. NAA is a multielement procedure; more than 25 elements can be measured in the sample at the same time.

Analysis can be performed on a sample as-received. In most cases, neither chemical treatment nor addition of reagent is required to prepare the sample for analysis; thus, contamination from excess sample handling and reagent addition is eliminated.

The required amount of sample (1µg to a few grams) is placed in a pre-cleaned polyethylene vial and sealed. Certified Standard Reference Materials obtained from either the National Institute of Standards and Technology or the U.S. Geological Survey are prepared in the same manner.

Samples and standards are then irradiated with neutrons in the Nuclear Engineering Teaching Laboratory TRIGA reactor. The radioactivity induced in the sample is compared to that of the standard.

The irradiation time, decay time (time between irradiation and counting), and counting time are a function of the element to be measured and can be optimized to maximize the induced activity and minimize interference from other elements.